The invention relates generally to wood-burning furnaces and the like, and more particularly to freestanding jacketed wood stoves adapted for maximum heating efficiency and safety.
Wood-burning heating stoves have been well known for a long time. As petroleum fuel became more available in the past, usage of wood stoves declined significantly. As a result, very few homes built in the last 75 years rely on wood stoves for heating. However, recent rapid increases in petroleum fuel costs have stimulated a resurgence in interest in the use of wood as heating fuel. Fireplaces are notoriously inefficient in heating a home. Therefore, many households, even those having fireplaces, are adopting wood stoves as a principal or back-up heat source.
Because many modern homes were not originally designed to accommodate wood stoves, use of conventional wood stoves in such homes presents a serious safety hazard. Most wood stoves rely principally upon radiation for transferring heat into an associated room. Hence, they tend to have extremely hot outer surfaces, typically 300.degree.-350.degree. F. This presents a safety hazard to occupants of the home who might inadvertently touch the stove. As a result, the number of wood stove-related burns has increased substantially in recent years.
Conventional wood stoves also create a fire hazard if the walls and floor adjacent the stove are not adequately protected from heat radiating from the stove. Adequate protection requires that a floor beneath the stove, and any wall adjacent the stove, be made of a material that is unaffected by intense radiate heat, such as brick or stone. Use of such materials adds significantly to the cost of retrofitting a home to receive a wood stove. A less expensive alternative, asbestos sheeting, is inadequate to protect the underlying floor from heat radiated by conventional wood stoves. Such stoves often produce floor temperatures more than 200.degree. F. above ambient temperatures. Moreover, asbestos is no longer readily available due to the recognition of health hazards in the manufacture and use of asbestos. Consequently, not realizing the dangers involved, or being unwilling to bear the additional expense, many homeowners have retrofitted their homes with wood stoves without taking adequate precautions. This has resulted in a substantial increase in home fire losses in recent years.
These safety problems are prompting more stringent regulation of wood stoves and how they are installed. Building code regulations affecting wood stoves are becoming more restrictive and are being policed more thoroughly in many parts of the country. Underwriting Laboratories (UL) standards for wood stoves were recently made more stringent as well. Compliance with the new UL standards is expected to be required in the future by both home insurers and local building regulatory agencies. Most conventional wood stoves fail to meet these standards. Hence, a wood stove capable of meeting these standards in needed.
Another problem is that existing wood stoves are generally less efficient than gas- or oil-fired heat furnaces. This is due in part to the fact that wood stoves typically heat radiatively. Consequently, a wood stove must generate more heat in order to adequately warm remote parts of the room or house in which it is situated. Also, existing wood stoves generally do not burn their fuel as completely as petroleum-fueled furnaces. A significant portion of the heat value of the wood escapes up the chimney in the form of unburnt gases. Finally, existing wood stoves are generally less efficient at extracting the heat produced by a fire. As a result, the cost advantages of burning wood over gas or oil are diminished.
An additional problem with existing wood stoves is that some of the unburnt gases condense in the chimney flue to form deposits of creosote. If creosote deposits are allowed to accumulate, a flue fire or explosion can occur. Such a fire or explosion can result in loss of the home in which the stove is situated. This danger can be avoided by cleaning the chimney flue periodically, but many existing stoves are extremely difficult to clean. It would be preferable if such deposits did not form at all. However, if such deposits do form, which is unavoidable with certain types of wood, it would be preferable if the stoves were arranged so that it would be very easy to clean the chimney flue without disconnecting and moving the stove.
A further problem is that unburnt gases emitted from the chimney pollute the air. Air pollution and smog due to burning of wood to heat homes has already become a significant problem in some areas. This problem is expected to worsen, if unchecked, as use of wood stoves increases. Consequently, several governmental air quality control agencies are investigating the possibility of regulating the use of wood stoves and the maximum level of pollutant emissions per stove. Hence, there is a need to reduce wood stove emissions of unburnt gases.
A wide variety of solutions to the above-described problems have been proposed. Many of these proposals have focused on modifying existing fireplaces to obtain some of the advantages of wood stoves without incurring excessive costs. One approach has been to install glass doors, either alone or in combination with some form of heat extractor and blower for circulating ambient air through enclosed tubes into contact with the fire or hot coals, and then expelling the air back into the room. Another approach has been to provide a jacketed fireplace insert such as those disclosed in U.S. Pat. Nos. 4,015,581 and 4,166,444 to Martenson. In such fireplace inserts, air is circulated into a jacketed airspace along the side, rear and top walls of the firebox to be warmed before returning to the room. However, such proposals fail to attain the efficiency of existing wood stoves.
Another proposed approach calls for a free-standing jacketed fireplace such as is disclosed in U.S. Pat. No. 2,703,567 to Manchester et al. This fireplace has a partially jacketed firebox and employs an upwardly inclined baffle or smoke shelf inside the firebox to deflect the flames toward the front of the firebox to radiate most of the heat forwardly into the room, rather than against the firebox walls. Consequently, little benefit is obtained from the jacketed airspace.
Another free-standing fireplace, proposed in U.S. Pat. No. 3,190,279 to Davis, has a surrounding enclosure and is mounted on a ventilated pedestal. However, the enclosure is not contoured to maximize heat extraction. It is spaced a substantial distance apart from the firebox walls so as to remain cool. A heat deflection plate inside the firebox helps to keep the firebox walls cool by deflecting the flames forwardly toward the front of the fireplace. The pedestal merely provides passive ventilation beneath the fireplace. It does nothing to contribute to the operation of the fireplace or to aid in circulating air into contact with the firebox walls.
U.S. Pat. No. 3,981,292 to Lilly et al. proposes a free-standing fireplace having heat tubes passing through an upper portion of the firebox from a plenum on one side, to outlet openings on the opposite side. A blower protruding from the rear of the fireplace blows ambient air through the plenum into the heat tubes to be heated before returning to the room. U.S. Pat. No. 4,150,658 to Woods discloses a wood stove of similar design. However, neither of these designs employs a jacket extending completely around the firebox. Moreover, the protruding blowers are unsightly and obstruct passage behind the apparatus.
Another form of wood stove is disclosed in U.S. Pat. No. 4,121,560 to Knight. The Knight patent discloses a firebox having a shield along the outer side of its rear wall and a rearwardly protruding blower mounted in an opening in the center of the shield to direct air into contact with the firebox and to circulate heated air around an associated room. However, the firebox is not completely jacketed. Its surfaces remain exposed. Also, no effort is made to maximize the efficiency of heat extraction from the firebox walls.
U.S. Pat. Nos. 4,092,976 and 4,147,153 to Buckner, disclose a partially jacketed wood stove having a blower for blowing air through an air space to hot air outlets. However, this design has several drawbacks. One drawback is the disadvantageous position of the rearwardly protruding blower. Another is that the hot radiant surfaces of the top and front walls are exposed, presenting a safety hazard. Moreover, the side and bottom walls of the jacket are likely to be too hot to safely touch or to permit positioning the stove near an unprotected wall or floor. A third drawback is that the stove requires a complex system of baffles to distribute airflow in the airspace. A fourth drawback is that the hot air outlets, positioned at the front of the stove on opposite sides of the doors, expell a concentrated blast of hot air forwardly into the room. As a result, regions along the lateral sides of the stove and at a distance therefrom are likely to be inadequately heated. Also, the air outlets are blocked whenever the doors are opened. A further drawback is that the door handles in such stoves can become too hot to touch. Simply substituting wooden handles does not solve this problem because the handles soon become scorched or charred.
U.S. Pat. No. 3,952,721 to Patterson also discloses a jacketed wood stove. This stove avoids some of the drawbacks of the Buckner design. However, the side, rear and bottom walls of the jacket are likely to become too hot to safely touch or to allow positioning of the stove near an unprotected floor or wall, but not hot enough to radiatively heat distant regions along the lateral sides of the stove. Warm air is expelled only from the front of the stove. Another drawback is that--although this design provides, to some extent, for progressive heating of air circulating through the airspace--it has no blower to take full advantage of progressive heating. Nor could such a blower be added to Patterson's design without radical changes. Finally, the hot frontal surfaces of the firebox are exposed, presenting a safety hazard. The door handle is also likely to become too hot to touch with bare hands.
Accordingly, there remains a need for a wood burning heating stove which can be safely installed and used in modern homes without incurring great expense, and which will efficiently extract heat from a wood fire and distribute the heat uniformly around the room in which the stove is situated.